High-frequency amplifier circuit



July 15 1952 M. L. THOMPSON 2,503,723

HIGH-FREQUENCY AMPLIFIER CIRCUIT Filed June s. 1947 l l 6km 0r 1 B+ B+ l'oLLow/Ns Suse i /7 Z4 2:/ :E Cnr/foo: C/Rculr l 1: 4/ lNaucrA/vc:

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CArHooe Clncu/r l lNvENToR l M\\.T.0N L". THOMPSON ATTORNEYS Patented .u-l'y 15, 1952 UNITEDl I .Y l lvlilt'qnL. Thompson, Huntingdon Valley; Pa., as# signor to Philco Corporation, va corporationl of Pennsylvania ApplicationJtune 3, 1947, Serial No. 752-',134j

'7Claims.

This invention relates in general tothe art of radio frequency signalA amplifiers, and more particularly to high frequency ampliers having comparatively high input resistance and adapted for operation in systems of high gain and selectivity.

The input resistance, that is, the control gridto-cathode resistance of an electrontube amplifier maybe dened-fas', -theratio of 'theY applied alternating grid-to'-cathodesignal voltage to v the component of gridcurrentinphase withy this voltage. lSuch grid current is inpart aresult of,thep capacitive -relation between thef grid. and othere tube electrodes -v including pthe; cathode. In ordinary low frequency-v'applications; the `input resistance of an electron tube. amplifier is so high asfto be vnegligible in' its Leifect asa load upon the input signalv sourceY Y At high frequencies ofgoperationas forV example, frequencies above -15 Inegacycles Midex-tendingthrough the present. day.V commercial television andfrequency, modulation spectrum to 108 megacycles, theinter-electrodecapacitances permit alternating -grideurrent flow suiiiciently large toreduce the effective input resistance-to a value which seriously v,affects lthe amplifier driving circuit.A As an lexample of such-operation, consider aradio .frequency amplier having a'tuned input circuit.V AVj-ijs is; Well understood, the .amplitude `of the g-ridhto-cathode signal voltage and the selectivity of the input circuit, thatfis; itsv ability to discriminate against signals other than the resonant frequency, are functions of the Q of the Atuned circuit. l

As the tuned circuit vis-.generally connected acrossthe grid and cathodefthe-input resistance ofnthe amplienas definedv above', isv a loss factor which limits themagnitudeof Q independently of the low-loss characteristics of the tuned circuit components. y

- An analytical studyofthefactors'influencing the input' resistance off anelectrontube circuit will benpresented hereinbelow; In general, however, the factors which Vinfluencethe input resistance of a pentode electronitube-ampiierlunder negative gridvoperation areithe-.transit-time of electronsg'andzthe, fact tha-t the lead'fromthe -cathode electrode. to the-'cathode pinonthebase lof the tube-isi common to2 thegridand theyplate lcircuits-oftheV tube. y

The inherent inductancefoflthis cathode lead has the electrical feiect oncausing the signal 'voltage' whichYV exists` acrossthe control grid-tojcatliodefcapacty to Adiffer: f1joxn-- the A-sigr'ialivoltage" :applied-to' they 'tube' from they inputl source circuit by an amount which'ssubstantially equal to the voltage drop across the lead i'nductance. In effect, the grid to cathode capacity isin series Withthe cathode leadinductance across the input circuit, Thevolt'age drop across the leadinductance` is appreciable since the cathode lead signal current causing' the drop' is the amplified plate current".v A

As a result of: the voltagedrop across the cathode lead inductancarthe signal current flowing through the capacity"betvve,en`gridy and cathode has acomponent inphase with the applied signal voltage. In vievv'of'fthe' aboveidefinition ofinput "resistance, it is thus evident that the cathoder lead inductanjce is in effect responsible for adding a load to a signal source voltage to be amplified. As will be shown below, thecomponents` ofinputv resistance caused'by both transit time and cathode inductanceare most seriou at the high frequencies'of operation.

There have heretoforeibeen developed`v a number of circuit arrangementsforeffectively neutralizing the inputv resistance resultingfrom the combined effects of transittime' and cathode lead inductance. In this connection; reference is ofthe cathode lead; that is, betwecnjthe cathode connecting pin on thetube'and'groundf- A capacitor is connected'betwee'n grid andground. The plate, screentgrid.and'suppressor of the tube are returnedto ground,v while. the grid. signal source is returnedto the -junction of; the added inductanceand the cathode. leadconnecting pin. If the components are selected so that theysatisfy a specied relation;` :thefinput resistance is substantiallyv infinite.

`'There'iav however.;v a fundamental disadvantage of the abcve-dcscribed ne'utralizingy circuit; namely,l that "the grid? returnV is toa point other than ground: In. radio. applications where the input Vsignal 'source -includes'a tuned .circuit comprising an inductance andvariableI condenser `in parallelythere are numerous advantagesrealized inY operating the variable. condenser? with' a grounded rotor. Such. operation .immediately precludes use of f the .above mentioned series". infplates and hasas a primary object the provision of a radio frequency amplifier of vsimplified overall construction having an extremely high input resistance. The principles;ofthepresent invention are applicable to substantially all high frequency amplifier systems, and in particular may be employed to neutralize input resistance in amplifiers having a common ground point for the input and output circuits thereof.

In brief, thepresent invention provides for the neutralization of the cathode lead inductance of an amplifier by capacitive means within the tube cathode circuit. It will be shown that by making the cathode circuit capacitive in eiect, at the operating frequency, the overall input resistance of the tube is increased and, if desired, may be actually made negative. In one embodiment of this invention, the cathode circuit ismade capacitive by series resonating the'inherent inductance of the cathode circuit .at afrequenoy above the operating frequency band. l

It is therefore another object of the present invention to provide a tuned radio frequency amplifier wherein'the input resistance is so high as yto have negligible effect upon the Q of the tuned components.

A further object of the present invention is to provide a high frequency 'amplier utilizing an electron tube cathode circuit which is capacitive at the operating frequency.

Another object of this invention is to provide a radio frequency amplifier wherein the inherent cathode lead inductance has been neutralized by a predetermined capacitor.

Still another object of the present invention is to provide -a high frequency amplifier having tuned'input and output circuits and which operates at'maximum overall gain with minimum input circuit loading.

A still further object of the present invention ,is to provide a high frequency tuned amplier particularly suited to operation above 15 megacycles and through a spectrum including a frequency above 108 megacycles, the latter frequency being the upper limit of present day commercial 'televisionand frequency modulation channels.

These and other objects of the present invention will now become apparent fromr the following detailed specification when taken in connection with the accompanying drawings in which:

Figure 1 is a schematic circuit diagram of a radio frequency amplifier incorporating the novel features of the present invention; and

Figure 2 is a schematicrcircuit diagram of a modified cathode circuit useful in certain application of the amplifier of Figure 1.

As previously mentioned, the input resistance of. a pentode electron tube may be regarded as thesum of the resistances caused by two factors; namely, the electron transit time within the tube from the cathode to the grid plane, and the inductance inherent in the cathode circuit of the tube. With reference again to Termans Radio Engineers Handbook? cited above, and more par- Y an antenna auto-transformer coil I4.

4 ticularly to pages 471 and 472 thereof, it is shown that the component of input resistance resulting from the nite interelectrode transit time (which resistance is herein designated as R1) may be stated approximately by:

The input resistance factor R1 caused by transit time phenomena is thus fixed by operating voltages, frequency, and tube geometry.

The input resistance component which results from the inductance of the cathode lead (which resistance is herein designated as R2) is shown by the Terman reference to be approximately:

1 Y R2-w2g'mLKCgk wherein:

w=21r (frequency) ,y

gm is the tube transconductance LK is the inherent inductance associated with the cathode lead of the tube Cgk is the grid-to-cathode capacity of the tube. a

The relation given by Equation 2 has been derived on the assumption that the reactance of the lead inductance Lx is small compared with the grid-cathode capacity Cgii.

It will be noted that the two components of input resistance have the same type of variation with respect to frequency and transconductance. In ordinary type pentode tubes, these effects are of the same order of magnitude. With the quantity f2 a factor in the denominator of each of these terms, it is evident that the input resistance decreases as a rapid function of frequency.

With the foregoing relationships in view, reference is now made to the drawings, and particularly Figure 1 thereof, for anlillustration of an amplifier circuit which embodies the principles ofthe present invention. In this figure. there is shown a radio frequency amplifier which is successfully applicable to frequency modulation, television. and the like signal amplification. In this example, the signal source comprises a dipole receiving antenna II coupled by' a transmission line I2 between ground and a tap I3 on Coil I4 is resonated at the desired: signal frequency by a parallel variable condenser I5, -the rotor plates of 'which are grounded. The coil I4 and condenser I5 thus comprise the receiver input tuned circuit, which as illustrated is coupled to thefcontrol grid of a pentode electron tube I6. The control grid of tube I6 is also connected through high resistance I'I to the automatic volume control '(AVC) or fixed bias source, vas the case maybe,

aeoa'zas a transformer' as input coil- I 4, andisacomponent of'a parallel resonant circuit-tuned by'variable condenser 24. vVariable condenser 24-isoperated with a grounded rotor and ismechanicallyganged to the input tuning condenserfl; themechanical connection being represented bythebroken line 25. As the rotors of both condensers Liv and 24 are grounded, it is clear that both input and output circuitsvofthe amplifier tube; IG- have a common ground point. The outputsignal'of'the tuned circuit comprising coil'23and condenser 24 is taken from a suitable tap 26l on coil 2-3"and applied to theA following stage of the receiver. Thetaps on coils |4 and' 23' are choseniso'that proper impedance `relationships are established.V

The cathode circuitof tube I6- is show-nlI to include an inductance 3 l, which itis emphasized, l

is not av physical external inductor; rather it. is

representative of the inherentand`v irreduciblecathodecircuit inductance resultingv in part fromA thevlead'within thetube I6 connecting-therethode electrode and the tube connecting pin, and inl part from the inductance addedas a result of tubevwiring and the like. Inductance 3| in the cathode circuit is lsimplya convenientmanner of'representing all inductance effec-ts ofthe tube cathode circuit. In commercially available tubes having indirectly heated cathodes, inducta-nce 3| has been found to'be of the order of 0.01 to 0.02 microhenry. At 100 megacycles, suchindu'ctance is equivalent-11:0 a reactance'between 6 andl 12 ohms.

In accordance with my' invention, a small capacitor` 32Y isadded in. series in the cathode circuit of tube 'lin Figure 1, and is shunted by a small resistor 3 3 to provide a'.V continuous direct ,current circuit vforthe tube. Thus, capacitor 32 andits shunting resistor- 33'v are in series between the cathode inductance 3l and ground. Y v

Withv reference to Equation 2 above, for the value of the input resistance dueto cathode lead inductance, itmay be seen thatfthe factorwLz; in thedenominator is, for the circuit-of1igure 1, the effective reactance of the-entire cathodecin cuit and that the relation for this resistance must be modified -to includethe capacitor 32. A Thus, re-writing- .Equation 2 and' consideringY the cathode circuit reactance: 'R2=" -l-1-- (3) .wg wL}{- w. vai

. i i-@L'fwvtif l' .v If. capacitor 32. is. somewhat larger than-,re- 'quired-:toi` establish4 resonance, ythe Vactin* Rz of the totalinputresistance.I

R'1Rj2 .Y RHR?.

will bepositive and; ofl hi'ghvalue. Iff'on the other hand, capacitor 32` isAA smaller than that required forresonance -at the operating frequency, R2 will be high and negative. lIf'ftz is negative andv largerlthan R1 thetotal input resistance Vwill be negative. Negative inputresis'tance in this case means that energy at signal frequency is fedA back into the 'grid circuit from thefplate supply. "f f 1' y The cathode circuit of tube 1 6 may be made capacitive forL negative input resistance by simply selecting capacitor 32 so that it series resonates inductance 3l at a frequency above the operating frequency band ofthe amplifier.

as in the moreconventional usage (although it will inevitably contribute to i the total bias); rather it is employed to provide a direct current path around capacitor 32. Similarly, capacitor32 differs very substantially, both in function and magnitude, from the usual R.F. by-passcapacitor customarilyl shunted across the usual cathode bias resistor. An' R.F. by-pass capacitor as; usually employed in combination with a vcon-1- ventional bias resistor (such as the 150 ohm yre` sistor, ycited above) would have a capacity of 0.01 to 0.05 uf., that'is,betwee n 10,000 and 50,000 wfg, 01 roughly about"v 1000 timesgreater than the capacitor 32 in the above example.

In general, inf thepre'sentinvention, the reactance of the capacitor 32 at the operating frequencygis not greater than the ohmic value of shunting resistor 3 3 and not less than one tenth of this ohmic value.

A cathode bias byfpass condenser would not therefore satisfy thev aforementioned resonance requirements for neutralization of the input resistance ofV tube l6.. In the circuit of Figure 1, vgrid bias is obtained through resistor Hfrom a fixed bias voltage supply (not shown) or an A VC bias sytem (not shown). 'Ivhe circuit of the Vamplifier is not dependent-upon resistor 33 for a bias potential.

The actual valueof capacitor 32 is a function ofthe frequencybandcovered by the amplifier. As a general 'rule, capacitor 32VVV will be of the order of 15 to '75 micromicrofarads,A which at megacycles covers a reactance range greater than that of thecathode inductance 3Ifas mentioneclYV above. ode to ground circuitis thus cz'rpacitive.v

An examination of Equation -3 will indicate `that as modified for consideration of lcapacitor 3 2, R2 is `no longer van inverse function ofthe frequency squared, asin R1 in Equation 1. cordingly neutralization'of the input resistance is not exact as the frequencyis varied. Over a narrow band of frequencies, however, as in the example cited above, 88 tor 108 megacyoles, such neutralization measurably improves amplifier operation, if thecapacitor 32 is chosenl to resonate the cathode' circuit above this band.

Although resistor 33 of -Figui'e'l' is 'not intended asa-bias source, thereis, as'has .been indicated above, an inevitabledireet currentfdrop across its terminals. fIfzer o' cathode bias operation Vis desirable, the cathode circuit arrangement of tube I 6; shownA "ini A Figure 2 'imayj be; employed.

It willbe-understoodthatFiguref2 lis only il'- The net reactance of the cath-- Connected as. thergrrespondinelv dsgnaidtub .4"1.; i. if. f f .As-rshqwgl Fleure- 2, tbe-,Cathede ind'utane y3 l... oigtube. .le-.is-resonaiedbyfa small. Capacitor .35.. Connected/.ig srQunsifand which' is.. Shunted .preerablysby a IQWQ 'inductancezor Chokeo 3E. For successful operation inaccordance vwith the abve-prinip1esihe values/off the Choke. 36 andcapaciioriare chosenfso ,that the, .parallel l capacitive over the frequency band O foperatlon, i 1 i Y Further, thef capacitive jeffect v.of the combinatiQn of...hekeand-apait0r ..35 :is Such that .thefathode indu-@tance iS resonaied above the operating frequencyband. ljAs the D..C. resistance-of chok e 3 6 lovv, the `dir ect currentidrop across it is negligible.- I'h e sole tube bias is the bias potential applied to the control grid, as in Figure 1. Y. f

In the utilization of the present invention as shown in Figures l and 2, the input resistance ofthe tube is increased ormade negative with the result that the Q ofV thetuned circuit cornprising coil I4 and variable condenser I5. (Figure 1.) is increasedimproving the tuning circuit selectivity and eifectively applying a higher signalvoltage-to the tube control grid. As a revSulifof the increased input resistance obtained Y necting the .ungrounded side of' transmission line l2 .to a tap-'I3Y which is nearer ground than would otherwise be usable. Such connection may .be employedto secure an increased antenna gain by a factor of tivoY or three, and has the further advantage of improving the image signal ratio of the amplifier input. Clearly, the principles herein described and illustrated'may be subject to various. other applications.` Itis thus preferred thatthe spirit and scope of this invention vbe dened solely by the appended claims.u .Iclaimf v 1. high frequencyamplier for amplifying signals comprising a circuit tunable over a predetermined -frequency band, said tunable circuit-lcomprisinga vacuum tube having atleast anode. cathode and control grid electrodes, a source of highfrequency signal energy having a pair of terminals, said control grid being connectedrto one of said terminals Aand said cathode being connected to the other of said ter.- .minals by Way of a cathode circuit, a load having a pair of terminals, said anode being connectedto one ofsaid load terminals and said cathode being connected tothe. other of said Yload terminals by Lway of said cathode circuit,

.8 nf. aidieba za .Signal output Circuit .including ncde-to-cathode circuit.v of said tube, the cathode; circ tof saidA tube beingr common to bth l said l ut ando utput circuits, said cathode nsing`l inductive yreactive components, nd; capacitive reactive meansi connected l 'in sries .with ai. leStSQm 9i Said reactive com- 'biehrireeueger Signalen-@vige a pairf Output thereby; 'being A@Qmmontoubojth grid and anode circuits ot said tube, said cathode circuit comprising inherent impedance having an excessive inductivejseries. component and means for neutralizing isaid Y inductive series component, said means j comprisingsa capacitor connected in saidqycathode circuit in series with said inductive component; and-.arranged to resonate with said inductive component at a frequency above .said frequencybandl .4.; A yhigh :frequency amplifier. comprising a circuit .t1inabla over aVAL predetermined frequency band, said tunable circuit comprising a vacuum tubahavine atleastanod,cathode. and controly gridfelectrodesfa grid-to-cathode input circuit, an anode-,to-cathodeutput circuit, the cathode 4portion of saidinputand output circuits being commento;- both, vsaid V4cathode, portion having vequivalr-nt. Series. inductance; andhavine equiva- .lentseries-aangename;such aste-resonate Said cathodeVY p.ortion; .at n a vfrequency above said frequencyband; -1-

Meansfg for increasing the input `.resistance yof a Y.high frequency. vacuum .tubeamplier having a cathode circuit common toits input` and output circuit, said cathode circuit having excessive equivalent seriesinductance, said means comprising a capacitor connected in said cathode circuit, said capacitor having a value such that .the equivalentl series capacitive reactance of said` cathode :.circuit' including Vsaid capacitor exceeds. 4said .equivalent .series inductive reactancef'. 1

p6.r Meansfor increasing ,the input kresistance oa "high frequency. vacuum-tube amplifier havinga'cathodecircuit common to its input and output circuit, :said cathode Zcircuit having excessivelequivalent'series inductance, said means comprisingacapacitor .connected in. said cathode circuit,;'s'aid.-capacitor'having a value such that the' equivalentseries capacitancefof said cathode circuiti'includi-ng: said jcapacitor exceeds said equivalenti seriesxinductance over .the operating range of saidgamplifier and Aresonates therewith at a frequency abover said range, and a radio i.frequency choke shunting'said capacitor.

'7. Means Yfor increasing the input resistance jofia high 'frequencyvacuum tube amplifier having a cathode-circuitfconimon Itoitsiinput and output circuit, said cathode'circuithaving excessive equivalent series inductance, said means comprising a capacitorjconnected in said caththerewith at a frequency above said range, and Number Name Date a direct current conductive means shuntng said 2,093,094 Peterson Sept. 14, 1937 capacitor and having a low impedance for d- 2,256,293 Salzberg Sept. 16, 1941 rect current and signals of 10W frequency and 2,261,203 Albright Nov. 4, 1941 having a high impedance for signals of high 10 2,293,262 Koch Aug. 18,1942 frequency. 2,271,519 Wolf Feb. .3, 1942 2,351,934 Kramoln June 20, 1944 9 10 ode circuit, said capacitor having a value such y REFERENCES CITED that the equivalent series capacitance of said The following references are of record in the cathode circuit including said capacitor exceeds me of this patent: said equivalent series industance over the operating range of said amplifier and resonates 5 UNITED STATESv PATENT MILTON L. THOMPSON. 2,373,184 Harold Apr. 10, 1945 

